When the sperm enters the space around the egg, a sperm-specific protein called Izumo on the sperm head binds to Juno receptors on the egg membrane. This binding triggers mechanisms to prevent multiple sperm from fertilizing the same egg (polyspermy), which would be lethal in humans.
Penetration of the corona radiata: Sperm release enzymes and use their flagella to pass through the outer layer of cells surrounding the egg
Penetration of the zona pellucida: Sperm bind to receptors on this protective coating and digest through it
Membrane fusion: The sperm and egg membranes fuse, allowing the sperm to enter.
The immediate result of sperm entry is to activate the oocyte to complete the second meiotic division and extrude the second polar body.
Nuclear fusion: The sperm nucleus fuses with the egg nucleus, enabling fusion of their genetic material
Once a sperm enters the egg, the egg immediately changes its surface to prevent other sperm from entering. The genetic material from the sperm (23 chromosomes) combines with the genetic material from the egg (23 chromosomes) to create a complete set of 46 chromosomes.
[This newly formed zygote contains all the genetic information needed to develop into a human being.
At fertilization, the diploid number of chromosomes is restored, the chromosomal sex of the new individual is determined and the ovum is activated to proceed to the first cleavage division.
The fertilization creates a single-celled zygote.